Elsewhere I have shown how the study of compound interest leads to the conception of a “growth curve” (the exponential curve) from whose properties we can derive, first, the theory of logarithms, then the conception of e as the limit of (1 + 1/n)n, and finally the differential coefficients of ax and logax. The method of differentiating ax and of introducing e to be described here follows entirely different lines, and may be useful to students who have approached logarithms in the ordinary way through the theory of indices and have begun the study of the calculus before reading what is commonly called “higher algebra.”